The effects of access region scaling on the performance of millimeter-wave GaN HEMTs is investigated through nanoscale carrier dynamics description obtained by full band Cellular Monte Carlo simulation. The drain current and transconductance have shown to increase monotonically up to respectively 5500 mA/mm and 1500 mS/mm by symmetrically scaling the source to gate and gate to drain distance from 635 nm to 50 nm. The electric field distribution has been studied for the shorter access regions and it was seen to be still far from the GaN breakdown limit. The access region scaling is found to greatly improve the frequency response of the device as well: from 340 GHz up to 860 GHz. Detailed simulation of the carrier dynamics in the area under the gate showed that these improvements are due to higher transit velocity of electrons at the source end of the gate.